This invention relates to a gas insulated switchgear and in particular to a protective system therefor.
FIG. 4 illustrates a conventional gas insulated switchgear. In the figure, the broken lines designate a housing having a gas medium for insulating a bus bar 2 within the gas chamber. When a fault condition occurs in a gas insulated bus bar 2, current transformers 4, 6 and 8 detect abnormal currents in the same phase. These abnormal currents are supplied to a control circuit 10, which causes the interrupters 12, 14 and 16 to be tripped to protect the bus bar 2.
On the other hand, when a fault condition occurs outside of the region to be protected for example, in a transmission line 24 in the air outside of bushings 18 where the bus bar 2 is connected to the transmission line 24, not all of the current transformers 4, 6 and 8 detect abnormal current in the same phase. Therefore, the control circuit 10 determines that the fault condition occurs outside of the region to be protected and does not protect the bus bar 2.
However, there are the following problems in the conventional protecting system. Since there is some error in the current transformer and the voltage transformer in a fault is within the region to be protected or the fault is outside of the region to be protected when the fault current is low and within the range of the error. Therefore, the conventional system is disadvantageous in that it is activated even when the fault is outside of the region to be protected or that it fails to achieve interrupting operation even when the fault occurs within the region to be protected.